Dysfunction of cobalamin metabolism can produce a form of deficiency which has as its clinical consequences methylmalonicacidemia, homocystinuria, mental retardation and megaloblastic anemia. The purpose of this program is to identify and characterize enzymes that catalyze the conversion of cyano- aqua- and methylcobalamin to the principal coenzyme form of the vitamin, adenosylcobalamin. The initial step in cobalamin metabolism is a complex transport process that involves external serum proteins known as transcobalamins. These proteins bind cobalamins and mediate their transport via specific plasma membrane receptors. The transcobalamins and other cobalamin binding proteins have been purified to near homogeneity and in high yield by a novel affinity chromatographic technique developed in this laboratory. The affinity matrix has also been useful in studying the receptor mediated transport process. Thus, in murine leukemia L1210 cells, receptors for transcobalamin appear to be localized on microvilli. Prokaryotes (e.g., Lactobacillus leichmannii) have an analogous transport system for cobalamins except that the external protein component for cobalamin binding is localized in the cell wall.